1. Field of the Invention
The present invention relates to a display device, and, more particularly, to a display device having superior driving characteristics and luminance characteristics.
2. Description of the Related Art
During fabrication of a thin film transistor (TFT) using polycrystalline silicon, bonding defects, such as dangling bonds existing on crystal grain boundaries of the polycrystalline silicon included in an active channel region, are known to function as traps for electric charge carriers.
Therefore, the size, size uniformity, number, position, and direction of crystal grains not only have a fatal effect upon TFT characteristics such as threshold voltage (Vth), subthreshold slope, charge carrier mobility, leakage current, and device stability, directly and indirectly, but these characteristics also have a fatal effect upon the uniformity of TFTs, depending on the position of the crystal grains during fabrication of an active matrix display substrate using TFTs.
The number of fatal crystal grain boundaries (hereinafter referred to as “primary” crystal grain boundaries) included in the active channel regions of TFTs on the whole substrate of a display device can be equivalent or different according to the size of the crystal grains, the inclination angle θ, the dimension of active channels (of length L and width W), and the position of each TFT on the substrate (FIG. 1A and FIG. 1B).
As illustrated in FIG. 1A and FIG. 1B, if the maximum number of crystal grain boundaries is Nmax, that is, the number of “primary” crystal grain boundaries included in the active channel regions for a size of crystal grains Gs, active channel dimension L×W, and inclination angle θ, the number of “primary” crystal grain boundaries included in the active channel regions according to a position of the TFT on a substrate or display device will be Nmax (3 in case of FIG. 1B) or Nmax-1 (2 in case of FIG. 1A), and the uniformity of the most excellent TFT characteristics can be secured when Nmax “primary” crystal grain boundaries are included in the active channel regions for all of the TFTs. That is, the more equal the number of crystal grain boundaries each TFT has, the more excellent uniformity a device obtains.
On the other hand, if the number of TFTs having Nmax “primary” crystal grain boundaries is equivalent to the number of TFTs having Nmax−1 “primary” crystal grain boundaries, it can be easily expected that uniformity is the worst in the characteristics of the TFTs on a TFT substrate or a display device.
As illustrated in FIG. 2A and FIG. 2B, polycrystalline, or single crystalline, particles are capable of forming large silicon grains on a substrate using the sequential lateral solidification (SLS) crystallization method, and it is reported that characteristics similar to the characteristics of TFTs fabricated of single crystalline silicon are obtained when fabricating a TFT using the large silicon grains.
However, numerous TFTs for driver and pixel arrays must be fabricated to fabricate an active matrix display. For example, approximately one million pixels are made in fabricating an active matrix display having SVGA resolution. One TFT is required in each pixel in the case of a liquid crystal display (LCD), and at least two or more TFTs per pixel are required in a display using an organic luminescent substance (for example, an organic electroluminescent device).
Therefore, it is impossible to fabricate TFTs by growing a certain number of crystal grains, in a certain direction only, for the active channel regions of each of one million to two million, or more, TFTs.
As a method to realize this, technology for transforming amorphous silicon on the whole substrate into polycrystalline silicon, or crystallizing the selected region only on the substrate by the SLS crystallization method after depositing amorphous silicon by PECVD, LPCVD, or sputtering, is disclosed referring to FIG. 2A and FIG. 2B, as disclosed in U.S. Pat. No. 6,322,625.
The selected region is also a considerably large region compared to an active channel region having a dimension of several μm×several μm. Furthermore, the size of the laser beam used in the SLS crystallization method is approximately several mm×dozens of mm, and the stepping and shifting of the laser beam or stage are essentially required to crystallize amorphous silicon of the whole region or selected region on a substrate, wherein misalignment between the regions on which the laser beam is irradiated exists. Therefore, the number of “primary” crystal grain boundaries included in the numerous active channel regions of the TFTs varies, and TFTs on the whole substrate, or in the driver region and the pixel cell region, have unpredictable nonuniformity. The nonuniformity has a fatal adverse effect on the embodiment of an active matrix display device.
Furthermore, it is disclosed in U.S. Pat. No. 6,177,391 that a barrier effect of crystal grain boundaries in the direction of the electric charge carrier is minimized, in the case that the direction of the active channels is parallel to the direction of the crystal grains grown by the SLS crystallization method when fabricating a TFT for LCD devices, including a driver and pixel array, by forming large silicon grains using the SLS crystallization method as illustrated in FIG. 3A. Therefore, TFT characteristics being second to single crystalline silicon can be obtained. On the other hand, crystal grain boundaries in which TFT characteristics act as a trap of the electric charge carriers exist, and TFT characteristics are greatly deteriorated, in the case that the direction of the active channels is perpendicular to the growing direction of the crystal grains, as illustrated in FIG. 3B.
Actually, there are cases in which TFTs in driver circuits are generally inclined to TFTs in pixel cell regions at an angle of 90° when fabricating an active matrix display, wherein the uniformity of the device can be improved by fabricating TFTs in such a way that the direction of the active channel regions is inclined to the crystal grain growing direction at an angle of 30 to 60° to improve the uniformity of characteristics between TFTs, as characteristics of each TFT are not greatly deteriorated, as illustrated in FIG. 3C.
However, there is a probability that fatal crystal grain boundaries are included in the active channel regions since this method also uses crystal grains having a limited size formed by the SLS crystallization method. Therefore, this method has problems in that unpredictable nonuniformity exists, causing differences of characteristics between the TFTs.
Furthermore, an item that should be considered along with the improvement of TFT characteristics is the uniformity of TFTs for driving pixels inside a panel.
It is difficult to realize uniform image quality of a display when characteristics are varied according to the position of a TFT in a substrate, even when the TFT shows good characteristics, particularly when the threshold voltage, that is, the turn-on voltage, of the TFT varies according to the position of the substrate.
Therefore, as a crystallization method that is capable of increasing the size of crystal grains and controlling the growing direction of the crystal grains, such as, for example, the SLS method, is developed, it is necessary to design and fabricate a TFT substrate that is suitable for the developed method.